L-amino acid producing bacterium

ABSTRACT

A method for producing an L-amino acid is described, for example, L-phenylalanine and L-histidine, by fermentation using a bacterium of the  Enterobacteriaceae  family, wherein the bacterium has been modified by attaching a DNA fragment able to be transcribed encoding the peptide represented in SEQ ID NO: 2, or a variant thereof, particularly a portion of the ssrA gene, to the 3′-end of gene encoding for the bacterial enzyme, which influences on the L-amino acid biosynthesis, such as chorismate mutase/prephenate dehydrogenase or phosphoglucose isomerase.

This application claims priority under 35 U.S.C. §119 to Russian Patent Application No. 2007135818 filed on Sep. 27, 2007. This document is incorporated in its entirety by reference. The Sequence Listing in electronic format filed herewith is also hereby incorporated by reference in its entirety (File Name: US-347_Seq_List; File Size: 34 KB; Date Created: Sep. 26, 2008).

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method for producing an L-amino acid by fermentation, and more specifically to genes which aid in this fermentation. These genes are useful for improving L-amino acid production, particularly L-phenylalanine and L-histidine.

2. Background Art

Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms obtained from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of L-amino acids. Many techniques to enhance L-amino acid production yields have been reported, including transformation of microorganisms with recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes of the feedback inhibition by the resulting L-amino acid (see, for example, WO 95/16042 or U.S. Pat. Nos. 4,346,170, 5,661,012 and 6,040,160) and creating bacterial strains which are deficient in the genes which use the precursors of the target compound for other pathways, or creating bacterial strains which are deficient in the genes responsible for degradation of the target compound.

These manipulations usually result in strains which cannot grow, grow only at a significantly reduced rate, or require additional nutrients, such as amino acids. For example, enhancing the expression of some genes may become excessive and could lead to significant inhibition of bacterial growth and, as a result, lower the ability of the bacterium to produce the target compound.

A possible approach for avoiding the difficulties described above is to optimize the expression of the genes which encode proteins involved in the distribution of the carbon, nitrogen, or phosphorus fluxes, or the excretion of the target substance out of the bacterial cell.

This aim was often accomplished by introducing a mutation into a promoter sequence of the amino acid- or nucleic acid-biosynthesizing genes (European Patent Application EP1033407A1), by obtaining the library of synthetic promoters with different strengths (Jensen P. R., and Hammer K., Appl. Environ. Microbiol., 1998, 64, No. 1. 82-87 Biotechnol. Bioeng., 1998, 58, 2-3, 191-5), and creating a library of artificial promoters (PCT application WO03089605).

It is known that tagging a protein with the ssrA peptide tag, encoded by SsrA RNA, marks the tagged protein for proteolytic degradation (Gottesmann S. and al, Genes & Dev., 12:1338-1347 (1998)). SsrA RNA (Synonyms: SsrA, tmRNA, 10Sa RNA, transfer-messenger RNA, SipB, B2621) functions to release a stalled ribosome from the end of a “broken” mRNA that is missing a stop codon by acting both as a tRNA and as an mRNA, using a “trans-translation” mechanism. SsrA also mediates the release of the stalled ribosome due to lack of an appropriate tRNA (Hayes C. S. and al, PNAS, 99(6):3440-3445 (2002)). Other translation problems can also lead to SsrA activity. In addition, SsrA RNA stimulates degradation of defective mRNAs (Yamamoto Y. et al, RNA, 9:408-418 (2003)).

The SsrA RNA contains a tRNA-like structural region that is processed by RNase P and charged by alanyl-tRNA synthetase (Komine Y et al, Proc. Natl. Acad. Sci. USA, 91 (20):9223-7 (1994)). The SsrA RNA also has a region that acts as a messenger RNA and encodes a translated tag that is added to the nascent protein and which targets this protein for degradation (Tu G.-F. and al, J Biol Chem, 270(16):9322-9326 (1995)). The structure of the SsrA RNA and the translational mechanism have been examined in detail (Corvaisier S. et al, J Biol Chem, 278(17):14788-97 (2003)). The SsrA RNA is transcribed as a larger precursor RNA that is then processed to form the mature RNA.

SsrA is similar to RNAs from Mycoplasma capricolum, Bacillus subtilis Ueda K. et al., Genes to Cells, 7(5): 509-519 (2000)), Dichelobacter nodosus, Synechococcus sp. strains PCC6301 and PCC6803, Thermus thermophilus, Salmonella enterica serovar Typhimurium, and to a two-piece RNA from Caulobacter crescentus. The tmRNA genes have been used as probes for the identification of bacterial species (Schonhuber W. and al, BMC Microbiology, 1(1):20 (2001)).

However, there have been no reports describing the use of ssrA-tagging for L-amino acid production, for example, L-phenylalanine or L-histidine production. Particularly, there is no previous description of using a bacterium of the Enterobacteriaceae family containing a DNA encoding the peptide of SEQ ID NO: 2 or a variant thereof, attached immediately downstream of a gene encoding a bacterial enzyme which influences L-amino acid biosynthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the scheme for tyrA gene extension.

FIG. 2 shows the relative positions of primers P1 and P2 on plasmid pMW118-attL-Cm-attR.

FIG. 3 shows the construction of the chromosomal DNA fragment with the deleted pheA gene.

FIG. 4 shows the scheme for replacing the cat gene locus with the pheA gene and the E. coli chromosome locus containing tyrA-ssaA allele and cat gene. The P4 and the P5 sites were used to check the construction.

FIG. 5 shows the alignment of the primary sequences of the tmRNA-encoded proteolysis-inducing peptide tag from Bacillus subtilis (BACSU—SEQ ID NO: 23), Bacillus stearothermophilus (BACST—SEQ ID NO: 24), Serratia marcescens (SERMA—SEQ ID NO: 25), Escherichia coli (ECOLI—SEQ ID NO: 26), Pseudomonas flurescens (PSEFL—SEQ ID NO: 27), Pseudomonas chlororaphis (PSECL—SEQ ID NO: 28), Pseudomonas putida (PSEUPU—SEQ ID NO: 29). The alignment was done by using the PIR Multiple Alignment program (pir.georgetown.edu). The identical amino acids are marked by an asterisk (*), similar amino acids are marked by a colon (:).

SUMMARY OF THE INVENTION

Aspects of the present invention include enhancing the productivity of L-amino acid-producing strains and a method for producing L-amino acids using these strains. The above aspects were achieved by finding that SsrA-tagging of an enzyme involved in the synthesis of a target amino acid that shares common steps in its biosynthesis pathway with other amino acids can enhance production of the target amino acid at the expense of the amino acids having common biosynthesis steps and/or precursors. It was also found that SsrA-tagging of an enzyme involved in the distribution of the carbon fluxes between different pathways of glycolysis (for example the pentose phosphate and Entner-Duodoroff pathways) can enhance production of those amino acids with precursors produced in one of the pathways of glycolysis. In either case, it was found that the prototrophic properties of the modified bacteria are maintained.

This aim was achieved by constructing a new variant tyrA gene and a new variant pgi gene, wherein the resulting proteins both have a C-terminal short peptide sequence encoded by part of ssrA gene. It was shown that the use of this mutant TyrA-ssrA could enhance L-phenylalanine production when the mutant tyrA-ssrA gene is introduced into the cells of the L-phenylalanine-producing strain in place of the native tyrA gene. It also was shown that the use of such mutant Pgi-ssrA could enhance L-histidine production when the mutant pgi-ssrA gene is introduced into the cells of the L-histidine-producing strain in place of the native pgi gene. Thus, the present invention has been completed.

It is an aspect of the present invention to provide an L-amino acid producing bacterium of the Enterobacteriaceae family comprising a DNA comprising a gene encoding a bacterial enzyme which influences L-amino acid biosynthesis, and a DNA fragment able to be transcribed and encoding the peptide of SEQ ID NO: 2, or a variant thereof, wherein said DNA fragment is attached to said gene at the 3′ end of said gene.

It is a further aspect of the present invention to provide the bacterium described above, wherein said bacterium belongs to the genus Escherichia.

It is a further aspect of the present invention to provide the bacterium described above, wherein said bacterium is an L-phenylalanine producing bacterium.

It is a further aspect of the present invention to provide the bacterium described above, wherein the enzyme is chorismate mutase/prephenate dehydrogenase.

It is a further aspect of the present invention to provide the bacterium described above, wherein said bacterium is an L-histidine producing bacterium.

It is a further aspect of the present invention to provide the bacterium described above, wherein the enzyme is phosphoglucose isomerase.

It is a further aspect of the present invention to provide a method for producing an L-amino acid comprising cultivating the bacterium described above in a culture medium, and isolating the L-amino acid from the culture medium.

It is a further aspect of the present invention to provide the method described above, wherein said L-amino acid is L-phenylalanine.

It is a further aspect of the present invention to provide the method described above, wherein said L-amino acid is L-histidine.

It is a further aspect of the present invention to provide a method for producing a lower alkyl ester of α-L-aspartyl-L-phenylalanine, comprising cultivating the bacterium described above in a culture medium to produce and accumulate L-phenylalanine in the medium, and synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from the aspartic acid or a derivative thereof and the obtained L-phenylalanine.

It is a further aspect of the present invention to provide the method described above, wherein the method further comprising esterifying L-phenylalanine to generate a lower alkyl ester of L-phenylalanine, condensing the lower alkyl ester of L-phenylalanine with the aspartic acid derivative, wherein the derivative is N-acyl-L-aspartic anhydride, separating the lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine from the reaction mixture, and hydrogenating the lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine to generate the lower alkyl ester of α-L-aspartyl-L-phenylalanine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Bacterium of the Present Invention

It is known that some L-amino acids have common precursors during biosynthesis by a cell, such as aromatic amino acids, branched chain amino acids, etc. When producing such an amino acid in a bacterial strain, it is useful to make the strain auxotrophic for the other amino acids which have a common precursor with the targeted amino acid. This can prevent the common precursor from directing the biosynthesis away from the biosynthetic pathway of the targeted amino acid.

Another method for ensuring production of the targeted amino acid, as opposed to those having a common precursor, is to generate a so called “leaky” mutation in an enzyme which is responsible for producing the other amino acids from the common precursor. In this case, it is not necessary to feed the bacterium with the other L-amino acid(s), since synthesis of these L-amino acids in the bacterium is suppressed.

SsrA-tagging of an enzyme results in a reduction in the enzyme activity, since the enzyme is degraded by the transcribed ssrA-tagged RNA via RNase P (Komine Y et al, Proc. Natl. Acad. Sci. USA, 91 (20):9223-7 (1994)) and proteolysis of the ssrA-tagged enzyme by ClpXP and ClpAP proteases (Wah D. A. et al, Chem. Biol., 9(11):1237-45 (2002)). Such modification by ssrA-tagging permits retention of the prototrophic properties of the modified bacterium, while exhibiting a leaky-type phenotype. Also, the ssrA-tagging permits the reduction of the level of by-products by decreasing the activity of one or several enzymes involved in the biosyntesis of such by-products.

“L-amino acid-producing bacterium” means a bacterium which has an ability to cause accumulation of an L-amino acid in a medium when the bacterium is cultured in the medium. The L-amino acid-producing ability may be imparted or enhanced by breeding. The phrase “L-amino acid-producing bacterium” as used herein also means a bacterium which is able to produce and cause accumulation of an L-amino acid in a culture medium in an amount larger than a wild-type or parental strain of E. coli, such as E. coli K-12, and preferably means that the microorganism is able to cause accumulation in a medium of an amount not less than 0.5 g/L, more preferably not less than 1.0 g/L of the target L-amino acid. The term “L-amino acids” includes L-alanine, L-arginine, L-asparagine, L-aspartic acid, L-cysteine, L-glutamic acid, L-glutamine, L-glycine, L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, L-tyrosine, and L-valine. L-histidine is particularly preferred. Aromatic amino acid, such as L-phenylalanine, L-tryptophan and L-tyrosine is more preferred and L-phenylalanine is particularly preferred.

The Enterobacteriaceae family includes bacteria belonging to the genera Escherichia, Enterobacter, Erwinia, Klebsiella, Pantoea, Providencia, Salmonella, Serratia, Shigella, Morganella, etc. Specifically, those classified into the Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (www.ncbi.nlm.nih.gov/htbinpost/Taxonomy/wgetorg?mode=Tree&id=1236&lvl=3&keep=1&srchmode=1&unlock) can be used. A bacterium belonging to the genus Escherichia or Pantoea is preferred.

The phrase “a bacterium belonging to the genus Escherichia” means that the bacterium is classified into the genus Escherichia according to the classification known to a person skilled in the art of microbiology. Examples of a bacterium belonging to the genus Escherichia as used in the present invention include, but are not limited to, Escherichia coli (E. coli).

The bacterium belonging to the genus Escherichia is not particularly limited; however, for example, bacteria described by Neidhardt, F. C. et al. (Escherichia coli and Salmonella typhimurium, American Society for Microbiology, Washington D.C., 1208, Table 1) are encompassed.

The bacterium encompasses a strain of the Enterobacteriaceae family which has an ability to produce an L-amino acid and has been modified by attaching a DNA fragment having part of the ssrA gene to the 3′-end of a gene encoding a bacterial enzyme. The part of the ssrA gene encodes for the peptide represented in SEQ ID NO: 2 or a variant thereof, and the bacterial enzyme is involved in the biosynthetic pathway of an L-amino acid which has a common precursor with the targeted L-amino acid. In addition, the bacterium encompasses a strain of the Enterobacteriaceae family which has an ability to produce an L-amino acid and has been transformed with a DNA fragment encoding part of the ssrA gene so that components of the peptide encoded by the DNA fragment are expressed.

The ssrA gene (synonyms: ECK2617, b2621, sipB) encodes the tmRNA (synonyms: SsrA, 10Sa RNA, transfer-messenger RNA, SipB, B2621). The ssrA gene (nucleotides from 2753615 to 2753977 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the smpB and intA genes on the chromosome of E. coli K-12. The ssrA gene from Escherichia coli is represented by SEQ ID NO: 1. The sequence of the tmRNA-encoded proteolysis-inducing peptide tag is represented by SEQ ID NO: 2. This peptide is encoded by nucleotides on positions from 90 to 119 of gene ssrA represented in SEQ ID NO: 1. Other ssrA genes have also been elucidated as follows: tmRNA-encoded proteolysis-inducing peptide tag from Bacillus subtilis, Bacillus stearothermophilus, Serratia marcescens, Escherichia coli, Pseudomonas flurescens, Pseudomonas chlororaphis, and Pseudomonas putida.

Bacteria which are able to concurrently produce two L-amino acids which have a common precursor may be used. The goal of the present invention was achieved by using a strain which had been previously modified to concurrently produce two amino acids, namely L-phenylalanine and L-tyrosine. Such a strain can be obtained by destroying the transcriptional dual regulator, namely, the tyrR gene by the standard Red-driven recombination method. Deletion of the tyrR gene can be confirmed by PCR using the chromosomal DNA of the transformed strain as a template. Bacteria which are able to produce L-amino acids may be used, and preferably bacteria which are able to concurrently produce phenylalanine and tyrosine E. coliMG1655 ΔtyrR is an example of such a strain. E. coli strain MG1655 ΔtyrR was obtained by substitution of the chromosomal tyrR gene with a DNA cassette containing the Cm marker followed by excision of the marker using standard techniques described in WO 05/010175. Also, an L-histidine producing strain in which the pgi gene is modified by ssrA-tagging to increase flux of D-glucose-6-phosphate to the pentose phosphate pathway to enhance productivity of L-histidine is also encompassed and described.

The phrase “a DNA fragment able to be transcribed and encoding the peptide of SEQ ID NO: 2 or a variant thereof, wherein said fragment is attached to said gene at the 3′ end of said gene” means that the gene encoding the bacterial enzyme has been modified to have additional nucleotide residues on its 3′-end as compared to a non-modified gene, for example, a gene of the wild-type strain. The presence of this DNA fragment (SEQ ID NO: 2 or a variant thereof) at the 3′-end of the gene encoding the bacterial enzyme leads to the formation of a tagged mRNA species, and the translated corresponding protein has an additional peptide tagged, or attached to, the C-terminus thereof. The bacterial enzyme may be modified by methods that include genetic recombining, C-terminal extensions, C-terminal fusions, and so forth. The length of the modified enzyme protein can be measured, for example, by Western blot with specific antibodies, protein sequencing, and the like. The phrase “attaching the DNA fragment encoding the peptide of SEQ ID NO: 2 or a variant thereof to the carboxyl terminus of the protein” can also mean that the translated modified protein is expressed in the bacterium in such a way that the modified protein is degraded at a controllable rate (McGinness K. E. and al, Molecular Cell, 22: 701-707 (2006)).

The phrase “a DNA fragment able to be transcribed” means that the DNA fragment is attached to the 3′-end of the gene coding for the bacterial enzyme in such way that the stop-codon of the gene encoding the bacterial enzyme is removed and the necessary DNA fragment is introduced immediately after the last codon of the gene. Such a DNA construct is transcribed as one transcriptional unit tagging the desired DNA fragment to the mRNA of the gene of interest (the bacterial enzyme). It also results in expression of ssrA-tagged proteins.

The phrase “a variant thereof” means a peptide which has changes in the sequence, whether they are deletions, insertions, additions, or substitutions of amino acids, but still maintains the desired activity at a useful level, for example, useful for decreasing the activity of tagged protein and consequently enhancing production of an L-amino acid. The number of changes in the variant peptide depends on the position or the type of amino acid residues in the primary structure of the peptide. The number of changes may be 1 to 4 and preferably 1 to 2 for the peptide listed as SEQ ID NO: 2. These changes in the variants can occur in regions of the peptide which are not critical for the function of the peptide. This is because some amino acids have high homology to one another so the activity is not affected by such a change. Therefore, the peptide variants may have a homology of not less than 70%, preferably not less than 80%, and more preferably not less than 90%, and most preferably not less than 95% with respect to the entire amino acid sequences shown in SEQ ID NO: 2 as long as the ssrA-tagged protein is recognized by proteases resulting in subsequent degradation. Homology between two amino acid sequences can be determined using the well-known methods, for example, the computer program BLAST 2.0, which calculates three parameters: score, identity and similarity.

The substitution, deletion, insertion, or addition of one or several amino acid residues should be conservative mutation(s) so that the activity is maintained. The representative conservative mutation is a conservative substitution. Examples of conservative substitutions include substitution of Ser or Thr for Ala, substitution of Gln, His or Lys for Arg, substitution of Glu, Gln, Lys, His or Asp for Asn, substitution of Asn, Glu or Gln for Asp, substitution of Ser or Ala for Cys, substitution of Asn, Glu, Lys, His, Asp or Arg for Gln, substitution of Asn, Gln, Lys or Asp for Glu, substitution of Pro for Gly, substitution of Asn, Lys, Gln, Arg or Tyr for His, substitution of Leu, Met, Val or Phe for Ile, substitution of Ile, Met, Val or Phe for Leu, substitution of Asn, Glu, Gln, His or Arg for Lys, substitution of Ile, Leu, Val or Phe for Met, substitution of Trp, Tyr, Met, Ile or Leu for Phe, substitution of Thr or Ala for Ser, substitution of Ser or Ala for Thr, substitution of Phe or Tyr for Trp, substitution of His, Phe or Trp for Tyr, and substitution of Met, Ile or Leu for Val.

Data comparing the primary sequences of tmRNA-encoded proteolysis-inducing peptide tag from Bacillus subtilis (BACSU), Bacillus stearothermophilus (BACST), Serratia marcescens (SERMA), Escherichia coli (ECOLI), Pseudomonas flurescens (PSEFL), Pseudomonas chlororaphis (PSECL), Pseudomonas putida (PSEUPU) show a high level of homology of the amino acid sequence “YALAA” (SEQ ID NO: 2) in the C-terminus (see FIG. 5). It is possible to substitute similar (marked by colon) amino acids residues by the similar amino acid residues without deterioration of the peptide activity. But modifications of other non-conserved amino acid residues may not lead to alteration of the activity of tmRNA-encoded proteolysis-inducing peptide tag. Peptide tagged amino acid sequences referred to by McGinness K. E. et al. (Molecular Cell, 22:701-707 (2006)) can also be considered as a peptide variant.

The DNAs which encode substantially the same peptides as components of the tag peptide may be obtained, for example, by modifying the nucleotide sequences of DNAs encoding components of tag peptide (SEQ ID NO: 1), for example, by means of the site-directed mutagenesis method so that one or more amino acid residues at a specified site involve deletion, substitution, insertion, or addition. DNAs modified as described above may be obtained by conventionally known mutation treatments. Such treatments include hydroxylamine treatment of the DNA encoding proteins, or treatment of the bacterium containing the DNA with UV irradiation or a reagent such as N-methyl-N′-nitro-N-nitrosoguanidine or nitrous acid. DNAs encoding substantially the same peptides as tag peptide can be obtained by expressing DNAs having a mutation as described above in an appropriate cell, and investigating the activity of the expressed product. DNAs encoding substantially the same peptide as tag peptide can also be obtained by isolating DNAs that are hybridizable with probes having nucleotide sequences which contain, for example, the nucleotide sequences shown in SEQ ID NO: 1 under the stringent conditions, and encode peptides having the activities of tag peptide. The “stringent conditions” referred to herein are conditions under which so-called specific hybrids are formed, and non-specific hybrids are not formed. For example, stringent conditions can be exemplified by conditions under which DNAs having high homology, for example, DNAs having homology of not less than 50%, preferably not less than 60%, more preferably not less than 70%, further preferably not less than 80%, and still more preferably not less than 90%, and most preferably not less than 95% are able to hybridize with each other, but DNAs having homology lower than the above are not able to hybridize with each other. Alternatively, stringent conditions may be exemplified by conditions under which DNA is able to hybridize at a salt concentration equivalent to ordinary washing conditions in Southern hybridization, i.e., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS, at 60° C. Duration of washing depends on the type of membrane used for blotting and, as a rule, what is recommended by the manufacturer. For example, recommended duration of washing, for example, for the Hybond™ N+ nylon membrane (Amersham), under stringent conditions approximately is 15 minutes. Preferably, washing may be performed 2 to 3 times.

Partial sequences of the nucleotide sequence of SEQ ID NO: 1 can also be used as probes. Probes may be prepared by PCR using primers based on the nucleotide sequence of SEQ ID NO: 1 and DNA fragments containing the nucleotide sequence of SEQ ID NO: 1 as templates.

The substitution, deletion, insertion, or addition of nucleotides as described above also includes mutations which naturally occur (mutant or variant), for example, due to variety in the species or genus of bacterium, which contains the components of tag peptide.

Expression of a bacterial gene which has a DNA fragment encoding the peptide represented in SEQ ID NO: 2, or a variant thereof, attached to the 3′-end of the gene can be achieved by transforming a bacterium with the DNA encoding the peptide, and more exactly, by introducing the DNA fragment into the bacterial chromosome downstream of the gene encoding the protein without disturbing the coding frame, or preferably, by replacing the native chromosomal gene with a mutant gene encoding the tagged protein by conventional methods. Transformation of a bacterium with DNA will result in the formation of a new chromosomal mutant gene encoding the elongated protein. Methods of transformation include any known methods that have hitherto been reported.

For example, the following methods may be employed to introduce the DNA encoding the mutant gene by gene recombination. A mutant gene is prepared, and the bacterium is transformed with a DNA fragment containing the mutant gene. Then, the native gene on the chromosome is replaced with the mutant gene by homologous recombination, and the resulting strain is selected. Such gene replacement by homologous recombination can be conducted by employing a linear DNA, which is known as “Red-driven integration” (Datsenko, K. A. and Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97, 12, p 6640-6645 (2000)), or by methods employing a plasmid containing a temperature-sensitive replication (U.S. Pat. No. 6,303,383 or JP 05-007491A). To increase the permeability of the cells to the DNA, treating the recipient cells with calcium chloride has been reported for Escherichia coli K-12 (Mandel, M. and Higa, A., J. Mol. Biol., 53, 159 (1970)) and may be used.

Methods for preparation of plasmid DNA include, but are not limited to, digestion and ligation of DNA, transformation, selection of an oligonucleotide as a primer and the like, or other methods well known to one skilled in the art. These methods are described, for instance, in Sambrook, J., Fritsch, E. F., and Maniatis, T., “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989).

In the present invention, the phrase “bacterial enzyme which influences L-amino acid biosynthesis” means an enzyme involved in the biosynthetic pathway of the undesired product, for example, the product which shares a common precursor with the targeted L-amino acid. The phrase “bacterial enzyme involved in the biosynthetic pathway of the undesired product” means an enzyme catalyzing the reaction of a common precursor of the targeted L-amino acid and the undesired product being converted into a the undesired product. This happens because the enzyme directs the reaction down the branch of the biosynthetic pathway which results in production of the undesired product. The phrase “bacterial enzyme which influences L-amino acid biosynthesis” also means an enzyme involved in the biosynthesis pathway of by-products of the targeted L-amino acid. The presence of such by-products can cause significant technical problems during purification, and negatively effect the L-amino acid production.

Genes encoding for the bacterial enzyme which influences L-amino acid biosynthesis are the genes of interest. Such genes are represented by, but not limited to, tyrA gene encoding for chorismate mutase/prephenate dehydrogenase, pgi gene encoding for phosphoglucose isomerase, ilvE gene encoding for branched-chain amino-acid aminotransferase, ilvA and tdcB genes encoding for threonine dehydratases, sdaA and sdaB genes encoding for L-threonine deaminases, argA gene encoding for N-acetylglutamate synthase, argG gene encoding for argininosuccinate synthase, proB gene encoding for γ-glutamyl kinase, thrB gene encoding for homoserine kinase, gene encoding for homoserine dehydrogenase etc. In general, any gene encoding an enzyme which causes the production of a metabolite resulting from deviation from the pathway of the target L-amino acid and/or involved in the formation of a by-product is the gene of interest.

Chorismate mutase/prephenate dehydrogenase (synonyms: B2600, TyrA) catalyzes reversible conversion between chorismate and prephenate, and reversible conversion between prephenate and p-hydroxyphenylpyruvate, which is the intermediate compound in the pathway biosynthesis of tyrosine. Chorismate conversion is the first step in the biosynthesis of both tyrosine and phenylalanine. Modification of the TyrA protein by ssrA-tagging can be useful for decreasing the flux of prephenate to the L-tyrosine biosynthetic pathway, and enhancing the production of L-phenylalanine. The activity of chorismate mutase/prephenate dehydrogenase can be measured by using a stopped-time assay in the presence of 2.5 mM chorismate or by immunodiffusion analysis of the enzyme-antibody complexes in plates prepared with 0.8% agarose (Rood J. I., Perrot B. et al., Eur J Biochem.; 124, 513-519 (1982)). The wild-type tyrA gene (synonyms: ECK2597, b2600) which encodes the chorismate mutase/prephenate dehydrogenase from Escherichia coli has been elucidated. The tyrA gene (nucleotides complementary to nucleotides 2736970 to 2738091 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the pheA and aroF genes on the chromosome of E. coli K-12. The tyrA gene from Escherichia coli is represented by SEQ ID NO: 3, and the amino acid sequence encoded by the tyrA gene is represented by SEQ ID NO: 4.

In the biosynthetic pathway of phenylalanine, reversible conversion between prephenate and phenylpyruvate is catalyzed by chorismate mutase/prephenate dehydratase (synonyms: B2599, PheA), which subunit is encoded by pheA gene (synonyms: ECK2596, b2599) in E. coli. The pheA gene (nucleotides from 2735767 to 2736927 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the pheA gene leader peptide and tyrA gene on the chromosome of E. coli K-12. The pheA gene from Escherichia coli is represented by SEQ ID NO: 5, and the amino acid sequence encoded by the pheA gene is represented by SEQ ID NO: 6. Simultaneous modification of the TyrA and PheA proteins by ssrA-tagging can be useful for decreasing the flux of chorismate to the L-tyrosine and L-tyrosine biosynthetic pathway and enhancing the production of L-phenylalanine.

Phosphoglucose isomerase (synonyms: B4025, Pgi, glucose-6-phosphate isomerase, D-glucose-6-phosphate-ketol-isomerase) catalyzes reversible conversion between β-D-glucose-6-phosphate and D-fructose-6-phosphate in gluconeogenesis, and glycolysis pathways. Decreasing phosphoglucose isomerase activity leads to the redistribution of carbon fluxes in favor of the pentose phosphate pathway in which histidine precursor metabolite 5-phosphoribosyl 1-pyrophosphate is generated. Activity of phosphoglucose isomerase can be detected by, for example, the method described in Friedberg I. (J Bacteriol., 112, 3:1201-1205 (1972)) in a coupled assay by using G6P dehydrogenase (Winkler H. H., J Bacteriol., 101, 2:470-475 (1970)). The wild-type pgi gene (synonyms: ECK4017, b4025) which encodes the glucose-6-phosphate isomerase from Escherichia coli has been elucidated. The pgi gene (nucleotides from 4,231,781 to 4,233,430 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the lysC and yjbE genes on the chromosome of E. coli K-12. The pgi gene from Escherichia coli is represented by SEQ ID NO: 7, and the amino acid sequence encoded by the pgi gene is represented by SEQ ID NO: 8.

Branched-chain amino acid aminotransferase (synonyms: B3770, IlvE) catalyzes a series of transamination reactions, each of which generates α-ketoglutarate and one of three aliphatic branched chain amino acids. Modification of the IlvE protein by ssrA-tagging can be useful for decreasing the flux of 2-ketoisovalerate to the L-valine biosynthetic pathway, which also results in decreasing the amount of L-isoleucine as by-product and enhancing the production of L-leucine. The activity of branched-chain amino acid aminotransferase can be measured by the method described, for example, in Lee-Peng F C et al (J. Bacteriol., 139(2); 339-45 (1979)). The wild-type ilvE gene (synonyms: ECK3762, b3770, threonine deaminase, L-threonine hydrolyase) which encodes the branched chain amino acid aminotransferase from Escherichia coli has been elucidated. The ilvE gene (nucleotides 3,950,507 to 3,951,436 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the ilvM and ilvD genes on the chromosome of E. coli K-12.

Threonine dehydratase (synonyms: B3772, Ile, IlvA) catalyzes the deamination of L-threonine. Modification of the IlvA protein by ssrA-tagging can be useful for preventing L-theonine degradation and decreasing the flux of L-threonine to the L-isoleucine biosynthetic pathway, as well as being useful for L-arginine production. The activity of threonine dehydratase can be measured by the method described, for example, in Eisenstein, E. (J. Biol. Chem., 266(9); 5801-7 (1991)). The wild-type ilvA gene (synonyms: ECK3764, b3772, ile) which encodes the threonine dehydratase from Escherichia coli has been elucidated. The ilvA gene (nucleotides 3,953,354 to 3,954,898 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the ilvD and ilvY genes on the chromosome of E. coli K-12.

Threonine dehydratase (synonyms: B3117, Tdc, TdcB, threonine deaminase, L-serine dehydratase, serine deaminase, L-threonine hydrolyase) catalyzes deamination of L-threonine. Modification of the TdcB protein by ssrA-tagging can be useful for preventing L-theonine degradation and decreasing the flux of L-threonine to the L-isoleucine biosynthetic pathway. The wild-type tdcB gene (synonyms: ECK3106, b3117, tdc) which encodes the threonine dehydratase from Escherichia coli has been elucidated. The tdcB gene (nucleotides complementary to nucleotides 3,263,061 to 3,264,050 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the tdcC and tdcA genes on the chromosome of E. coli K-12.

Threonine deaminases SdaA (synonyms: B1814, SdaA, SDH1, L-SD, L-threonine deaminase I, L-serine dehydratase 1, SDH-1, L-SD1, L-hydroxyaminoacid dehydratase 1, L-serine deaminase 1, L-serine hydro-lyase 1) and SdaB (synonyms: B2797, SdaB, L-threonine deaminase II, L-serine deaminase 2, L-serine dehydratase 2, SDH-2, L-SD2, L-serine hydrolyase 2) catalyze deamination of L-threonine. Modification of the threonine deaminase protein by ssrA-tagging can be useful for preventing L-theonine degradartion and decreasing the flux of L-threonine to the L-isoleucine biosynthetic pathway. The wild-type sdaA gene (synonyms: ECK1812, b1814) and sdaB gene (synonyms: ECK2792, b2797) which encode the threonine deaminases from Escherichia coli have been elucidated. The sdaA gene (nucleotides 1,894,956 to 1,896,320 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the yeaB and yoaD genes on the chromosome of E. coli K-12. The sdaB gene (nucleotides 2,927,598 to 2,928,965 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the sdaC and xni genes on the chromosome of E. coli K-12.

N-acetylglutamate synthase (synonyms: B2818, ArgA, NAGS, acetyl-CoA:L-glutamate N-acetyltransferase, amino-acid-N-acetyltransferase) catalyzes the synthesis of N-acetylglutamate from L-glutamate and acetyl-CoA, which is the first step in the superpathway of arginine and ornithine biosynthesis. Modification of the ArgA protein by ssrA-tagging can be useful to accumulate glutamate for L-proline production. The activity of N-acetylglutamate synthase can be measured by the method described, for example, in Marvil, D. K. and Leisinger, T. (J. Biol. Chem., 252(10); 3295-303 (1977)). The wild-type argA gene (synonyms: ECK2814, Arg2, Arg1, b2818) which encodes the N-acetylglutamate synthase from Escherichia coli has been elucidated. The argA gene (nucleotides 2,947,264 to 2,948,595 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the amiC and recD genes on the chromosome of E. coli K-12.

Argininosuccinate synthase (synonyms: B3172, ArgG, argininosuccinate synthetase, citrulline-aspartate ligase, L-citrulline:L-aspartate ligase) catalyzes the synthesis of argininosuccinate L-aspartate and citrulline. Modification of the ArgG protein by ssrA-tagging can be useful for citrulline production. The wild-type argG gene (synonyms: ECK3161, b3172) which encodes the argininosuccinate synthase from Escherichia coli has been elucidated. The argG gene (nucleotides 3,316,659 to 3,318,002 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the argR and yhbX genes on the chromosome of E. coli K-12.

γ-glutamyl kinase (synonyms: B0242, ProB, glutamate 5-kinase, GK, ATP:L-glutamate 5-phosphotransferase, G5K) catalyzes the reaction of glutamate phosphorylation, which is the first reaction in the synthesis of proline. Modification of the ProB protein by ssrA-tagging can be useful to accumulate glutamate for L-arginine production. The activity of γ-glutamyl kinase can be measured by the method described, for example, in Smith, C. J. et al, (J. Bacteriol., 157(2); 545-51 (1984)). The wild-type proB gene (synonyms: ECK0243, pro(2), b0242, pro2) which encodes the γ-glutamyl kinase from Escherichia coli has been elucidated. The proB gene (nucleotides 259,612 to 260,715 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the phoE and proA genes on the chromosome of E. coli K-12.

Homoserine kinase (synonyms: B0003, ThrB, ATP:L-homoserine O-phosphotransferase) catalyzes the reaction of homoserine phosphorylation. Modification of the ThrB protein by ssrA-tagging can be useful for L-methionine production. The activity of homoserine kinase can be measured by the method described, for example, in Shames, S. L. and Wedler, F. C. (Arch. Biochem. Biophys., 235(2); 359-70 (1984)). The wild-type thrB gene (synonyms: ECK0003, b0003) which encodes the homoserine kinase from Escherichia coli has been elucidated. The thrB gene (nucleotides 2,801 to 3,733 in the sequence of GenBank accession NC_000913.2, gi: 49175990) is located between the thrA and thrC genes on the chromosome of E. coli K-12.

Homoserine dehydrogenase catalyzes the reaction of the formation of L-aspartate-semialdehyde from homoserine. In Escherichia coli, the homoserine dehydrogenase is the part of aspartate kinase/homoserine dehydrogenase protein encoded by thrA gene. But in other microorganisms, such as Corynebacteria, homoserine dehydrogenase is a separate enzyme. Modification of the homoserine dehydrogenase by ssrA-tagging can be useful for L-lysine production.

The ssrA, tyrA, pheA, pgi, ilvE, ilvA, tdcB, sdaA, sdaB, argA, argG, proB, thrB, and other genes of interest can be obtained by PCR (polymerase chain reaction; refer to White, T. J. et al., Trends Genet., 5, 185 (1989)) utilizing primers prepared based on the known nucleotide sequences of the genes. Genes encoding for tag peptides from other microorganisms can be obtained in a similar manner.

The above-described techniques and teachings for attaching a DNA fragment able to be transcribed encoding the peptide represented in SEQ ID NO: 2 or a variant thereof, to the 3′-end of the genes encoding chorismate mutase/prefenate dehydrogenase and phosphoglucose isomerase can be similarly applied to modify the activities of other proteins transcribed from other genes of interest.

The bacterium can be obtained by introducing the aforementioned DNAs into a bacterium which inherently have the ability to produce L-amino acids. Alternatively, the bacterium can be obtained by imparting the ability to produce L-amino acids to a bacterium which already contains the DNAs.

L-Amino Acid-Producing Bacteria

Bacteria which are able to produce either aromatic or non-aromatic L-amino acids may be used in the present invention.

L-Phenylalanine-Producing Bacteria

Examples of parent strains for deriving L-phenylalanine-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli AJ12739 (tyrA::Tn10, tyrR) (VKPM B-8197); E. coli HW1089 (ATCC 55371) harboring the mutant pheA34 gene (U.S. Pat. No. 5,354,672); E. coli MWEC101-b (KR8903681); E. coli NRRL B-12141, NRRL B-12145, NRRL B-12146 and NRRL B-12147 (U.S. Pat. No. 4,407,952). Also, as a parent strain, E. coli K-12 [W3110 (tyrA)/pPHAB (FERM BP-3566), E. coli K-12 [W3110 (tyrA)/pPHAD] (FERM BP-12659), E. coli K-12 [W3110 (tyrA)/pPHATerm] (FERM BP-12662) and E. coli K-12 [W3110 (tyrA)/pBR-aroG4, pACMAB] named as AJ 12604 (FERM BP-3579) may be used (EP 488424 B1). Furthermore, L-phenylalanine producing bacteria belonging to the genus Escherichia with an enhanced activity of the protein encoded by the yedA gene or the yddG gene may also be used (U.S. patent applications 2003/0148473 A1 and 2003/0157667 A1).

L-Histidine-Producing Bacteria

Examples of parent strains for deriving L-histidine-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli strain 24 (VKPM B-5945, RU2003677); E. coli strain 80 (VKPM B-7270, RU2119536); E. coli NRRL B-12116-B12121 (U.S. Pat. No. 4,388,405); E. coli H-9342 (FERM BP-6675) and H-9343 (FERM BP-6676) (U.S. Pat. No. 6,344,347); E. coli H-9341 (FERM BP-6674) (EP1085087); E. coli A180/pFM201 (U.S. Pat. No. 6,258,554), and the like.

Examples of parent strains for deriving L-histidine-producing bacteria also include strains in which expression of one or more genes encoding an L-histidine biosynthetic enzyme are enhanced. Examples of such genes include genes encoding ATP phosphoribosyltransferase (hisG), phosphoribosyl AMP cyclohydrolase (hisI), phosphoribosyl-ATP pyrophosphohydrolase (hisIE), phosphoribosylformimino-5-aminoimidazole carboxamide ribotide isomerase (hisA), amidotransferase (hisH), histidinol phosphate aminotransferase (hisC), histidinol phosphatase (hisB), histidinol dehydrogenase (hisD), and so forth.

It is known that the L-histidine biosynthetic enzymes encoded by hisG and hisBHAFI are inhibited by L-histidine, and therefore an L-histidine-producing ability can also be efficiently enhanced by introducing a mutation conferring resistance to the feedback inhibition into ATP phosphoribosyltransferase (Russian Patent Nos. 2003677 and 2119536).

Specific examples of strains having an L-histidine-producing ability include E. coli FERM-P 5038 and 5048 which have been introduced with a vector carrying a DNA encoding an L-histidine-biosynthetic enzyme (JP 56-005099 A), E. coli strains introduced with rht, a gene for an amino acid-export (EP1016710A), E. coli 80 strain imparted with sulfaguanidine, DL-1,2,4-triazole-3-alanine, and streptomycin-resistance (VKPM B-7270, Russian Patent No. 2119536), and so forth.

L-Tryptophan-Producing Bacteria

Examples of parent strains for deriving the L-tryptophan-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli JP4735/pMU3028 (DSM10122) and JP6015/pMU91 (DSM10123) which is deficient in the tryptophanyl-tRNA synthetase encoded by mutant trpS gene (U.S. Pat. No. 5,756,345); E. coli SV164 (pGH5) having a serA allele encoding phosphoglycerate dehydrogenase free from feedback inhibition by serine and a trpE allele encoding anthranilate synthase not subject to feedback inhibition by tryptophan (U.S. Pat. No. 6,180,373); E. coli AGX17 (pGX44) (NRRL B-12263) and AGX6(pGX50)aroP (NRRL B-12264) deficient in the enzyme tryptophanase (U.S. Pat. No. 4,371,614); E. coli AGX17/pGX50,pACKG4-pps in which a phosphoenolpyruvate-producing ability is enhanced (WO9708333, U.S. Pat. No. 6,319,696), and the like may be used. L-tryptophan-producing bacteria belonging to the genus Escherichia with an enhanced activity of the identified protein encoded by and the yedA gene or the yddG gene may also be used (U.S. patent applications 2003/0148473 A1 and 2003/0157667 A1).

Examples of parent strains for deriving the L-tryptophan-producing bacteria also include strains in which one or more activities of the enzymes selected from anthranilate synthase, phosphoglycerate dehydrogenase, and tryptophan synthase are enhanced. The anthranilate synthase and phosphoglycerate dehydrogenase are both subject to feedback inhibition by L-tryptophan and L-serine, so that a mutation desensitizing the feedback inhibition may be introduced into these enzymes. Specific examples of strains having such a mutation include a E. coli SV164 which harbors desensitized anthranilate synthase and a transformant strain obtained by introducing into the E. coli SV164 the plasmid pGH5 (WO 94/08031), which contains a mutant serA gene encoding feedback-desensitized phosphoglycerate dehydrogenase.

Examples of parent strains for deriving the L-tryptophan-producing bacteria also include strains into which the tryptophan operon which contains a gene encoding desensitized anthranilate synthase has been introduced (JP 57-71397 A, JP 62-244382 A, U.S. Pat. No. 4,371,614). Moreover, L-tryptophan-producing ability may be imparted by enhancing expression of a gene which encodes tryptophan synthase, among tryptophan operons (trpBA). The tryptophan synthase consists of α and β subunits which are encoded by the trpA and trpB genes, respectively. In addition, L-tryptophan-producing ability may be improved by enhancing expression of the isocitrate lyase-malate synthase operon (WO2005/103275).

L-Valine-Producing Bacteria

Example of parent strains for deriving L-valine-producing bacteria of, but are not limited to, strains which have been modified to overexpress the ilvGMEDA operon (U.S. Pat. No. 5,998,178). It is desirable to remove the region of the ilvGMEDA operon which is required for attenuation so that expression of the operon is not attenuated by L-valine that is produced. Furthermore, the ilvA gene in the operon is desirably disrupted so that threonine deaminase activity is decreased.

Examples of parent strains for deriving L-valine-producing bacteria also include mutants having a mutation of amino-acyl t-RNA synthetase (U.S. Pat. No. 5,658,766). For example, E. coli VL1970, which has a mutation in the ileS gene encoding isoleucine tRNA synthetase, can be used. E. coli VL1970 has been deposited in the Russian National Collection of Industrial Microorganisms (VKPM) (Russia, 117545 Moscow, 1^(st) Dorozhny Proezd, 1) on Jun. 24, 1988 under accession number VKPM B-4411.

Furthermore, mutants requiring lipoic acid for growth and/or lacking H⁺-ATPase can also be used as parent strains (WO96/06926).

L-Isoleucine-Producing Bacteria

Examples of parent strains for deriving L-isoleucine producing bacteria include, but are not limited to, mutants having resistance to 6-dimethylaminopurine (JP 5-304969 A), mutants having resistance to an isoleucine analogue such as thiaisoleucine and isoleucine hydroxamate, and mutants additionally having resistance to DL-ethionine and/or arginine hydroxamate (JP 5-130882 A). In addition, recombinant strains transformed with genes encoding proteins involved in L-isoleucine biosynthesis, such as threonine deaminase and acetohydroxate synthase, can also be used as parent strains (JP 2-458 A, FR 0356739, and U.S. Pat. No. 5,998,178).

L-Leucine-Producing Bacteria

Examples of parent strains for deriving L-leucine-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli strains resistant to leucine (for example, the strain 57 (VKPM B-7386, U.S. Pat. No. 6,124,121)) or leucine analogs including β-2-thienylalanine, 3-hydroxyleucine, 4-azaleucine, 5,5,5-trifluoroleucine (JP 62-34397 B and JP 8-70879 A); E. coli strains obtained by the gene engineering method described in WO96/06926; E. coli H-9068 (JP 8-70879 A), and the like.

The bacterium may be improved by enhancing the expression of one or more genes involved in L-leucine biosynthesis. Examples include genes of the leuABCD operon, which are preferably represented by a mutant leuA gene coding for isopropylmalate synthase which is not subject to feedback inhibition by L-leucine (U.S. Pat. No. 6,403,342). In addition, the bacterium may be improved by enhancing the expression of one or more genes coding for proteins which excrete L-amino acid from the bacterial cell. Examples of such genes include the b2682 and b2683 genes (ygaZH genes) (EP 1239041 A2).

L-Arginine-Producing Bacteria

Examples of parent strains for deriving L-arginine-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia, such as E. coli strain 237 (VKPM B-7925) (U.S. Patent Application 2002/058315 A1) and its derivative strains harboring mutant N-acetylglutamate synthase (Russian Patent Application No. 2001112869), E. coli strain 382 (VKPM B-7926) (EP1170358A1), an arginine-producing strain into which argA gene encoding N-acetylglutamate synthetase is introduced therein (EP1170361A1), and the like.

Examples of parent strains for deriving L-arginine producing bacteria also include strains in which expression of one or more genes encoding an L-arginine biosynthetic enzyme are enhanced. Examples of such genes include genes encoding N-acetylglutamyl phosphate reductase (argC), ornithine acetyl transferase (argJ), N-acetylglutamate kinase (argB), acetylornithine transaminase (argD), ornithine carbamoyl transferase (argF), argininosuccinic acid synthetase (argG), argininosuccinic acid lyase (argH), and carbamoyl phosphate synthetase (carAB).

L-Citrulline-Producing Bacteria

Examples of parent strains for deriving L-citrulline-producing bacteria include, but are not limited to, strains belonging to the genus Escherichia containing mutant feedback resistant carbamoylphosphate synthetase (U.S. Pat. No. 6,991,924) and the like.

2. Method of the Present Invention

The method of the present invention is a method for producing an L-amino acid by cultivating the bacterium in a culture medium to produce and excrete the L-amino acid into the medium, and collecting the L-amino acid from the medium.

The cultivation, collection, and purification of an L-amino acid from the medium and the like may be performed in a manner similar to conventional fermentation methods wherein an amino acid is produced using a bacterium.

A medium used for culture may be either a synthetic or natural medium, so long as the medium includes a carbon source and a nitrogen source and minerals and, if necessary, appropriate amounts of nutrients which the bacterium requires for growth. The carbon source may include various carbohydrates such as glucose and sucrose, and various organic acids. Depending on the mode of assimilation of the used microorganism, alcohol, including ethanol and glycerol, may be used. As the nitrogen source, various ammonium salts such as ammonia and ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate, and digested fermentative microorganism can be used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like can be used. As vitamins, thiamine, yeast extract, and the like, can be used.

The cultivation is preferably performed under aerobic conditions, such as a shaking culture, and a stirring culture with aeration, at a temperature of 20 to 40° C., preferably 30 to 38° C. The pH of the culture is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, a 1 to 5-day cultivation leads to accumulation of the target L-amino acid in the liquid medium.

After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and then the L-amino acid can be collected and purified by ion-exchange, concentration, and/or crystallization methods.

Phenylalanine produced by the method may be used for, for example, producing the lower alkyl ester of α-L-aspartyl-L-phenylalanine (also referred to as “aspartame”). That is, the method includes production of the lower alkyl ester of α-L-aspartyl-L-phenylalanine by using L-phenylalanine as a raw material. The method includes synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from L-phenylalanine produced by the method as described above and aspartic acid or its derivative. As a lower alkyl ester, methyl ester, ethyl ester and propyl ester, or the like can be mentioned.

The process for synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from L-phenylalanine and aspartic acid or its derivative is not particularly limited and any conventional method can be applied so long as L-phenylalanine or its derivative can be used for synthesis of the lower alkyl ester of α-L-aspartyl-L-phenylalanine. Specifically, for example, the lower alkyl ester of α-L-aspartyl-L-phenylalanine may be produced by the following process (U.S. Pat. No. 3,786,039). L-phenylalanine is esterified to obtain the lower alkyl ester of L-phenylalanine. The L-phenylalanine alkyl ester is then reacted with L-aspartic acid derivative in which the amino group and .beta.carboxyl group are protected, and the a-carboxyl group is esterified to activate. The derivative includes N-acyl-L-aspartic anhydride such as N-formyl-, N-carbobenzoxy-, or N-p-methoxycarbobenzoxy-L-aspartic anhydride. By the condensation reaction, a mixture of N-acyl-α-L-aspartyl-L-phenylalanine and N-acyl-α-L-aspartyl-L-phenylalanine is obtained. If the condensation reaction is performed in the presence of an organic acid having an acid dissociation constant at 37° C. of 10−4 or less, the ratio of the α form to the β form in the mixture is increased (Japanese Patent Laid-Open Publication No. 51-113841). Then the N-acyl-α-L-aspartyl-L-phenylalanine is separated from the mixture, followed by hydrogenation to obtain α-L-aspartyl-L-phenylalanine.

EXAMPLES

The present invention will be more concretely explained below with reference to the following non-limiting Examples.

Example 1 Construction of the E. coli Strain MG1655ΔtyrRtyrA-ssrA, which Contains the Mutant tyrA-ssrA Gene

1. Substitution of the Native pheA Gene and 36-nt Located at the 3′ End of tyrA Gene Region, in E. coli with the Mutant tyrA-ssrA Gene.

To substitute the native pheA gene and 36-nt located at the 3′ end of tyrA gene region, a DNA fragment carrying 1) the 36-nt located at the 5′ end of pheA gene, 2) cat gene with attL and attR sites, 3) a DNA fragment encoding the chloramphenicol resistance marker (Cm^(R)), 4) the stop codon TAA, 5) the 30-nt located at positions from 90 to 119 of the ssrA gene, and 6) the 36-nt located at the 3′ end of tyrA gene was integrated into the chromosome of E. coli MG1655 ΔtyrR in place of the native pheA gene and the 3′-end tyrA gene region by the method described by Datsenko K. A. and Wanner B. L. (Proc. Natl. Acad. Sci. USA, 2000, 97, 6640-6645) which is also called “Red-mediated integration” and/or “Red-driven integration” (FIG. 1).

This DNA fragment was obtained in two consecutive PCR runs where 5 μl of amplification products from the first PCR were used as a template for the second PCR.

The first DNA fragment containing the Cm^(R) marker encoded by the cat gene was obtained by PCR using primers P1 (SEQ ID NO: 9) and P2 (SEQ ID NO: 10) and plasmid pMW118-attL-Cm-attR as a template (WO 05/010175) (FIG. 2). Primer P1 contains both a region identical to the 36-nt region located at the 5′ end of the pheA gene and a region complementary to the 27-nt attL region of pMW118-attL-Cm-attR. Primer P2 contains both a region identical to the 30-nt region of the ssrA gene and a region complementary to the 27-nt attR region of pMW118-attL-Cm-attR. Between these two regions, the stop codon TAA was inserted in primer P2. Conditions for PCR were as follows: denaturation for 5 min at 95° C.; profile for 30 cycles: 30 sec at 95° C., 30 sec at 50° C., 1 min at 72° C.; final step: 5 min at 72° C.

A 1709-bp PCR product was obtained and purified in agarose gel and 5 μl of this was used as a template for the second PCR reaction, using primers P1 (SEQ ID NO: 9) and P3 (SEQ ID NO: 11). Primer P3 contains both a region complementary to the 36-nt region located at the 3′ end of the tyrA gene and a region identical to the 18-nt region located at the 5′-end of the primer P2. Conditions for PCR were as follows: denaturation for 5 min at 95° C.; profile for 30 cycles: 30 sec at 95° C., 30 sec at 50° C., 1 min at 72° C.; final step: 5 min at 72° C.

A 1745-bp PCR product was obtained and purified in agarose gel and was used for electroporation of the E. coli strain MG1655ΔtyrR, which contains the plasmid pKD46 which has a temperature-sensitive replication origin. The plasmid pKD46 (Datsenko, K. A. and Wanner, B. L., Proc. Natl. Acad. Sci. USA, 2000, 97:12:6640-45) includes a 2,154 nucleotide DNA fragment of phage λ (nucleotide positions 31088 to 33241, GenBank accession no. J02459), and contains genes of the λ Red homologous recombination system (γ, β, exo genes) under the control of the arabinose-inducible P_(araB) promoter. The plasmid pKD46 is necessary for integration of the PCR product into the chromosome of the E. coli strain MG1655ΔtyrR in the place of the native pheA gene and 36-nt located at the 3′ end of tyrA gene region.

Electrocompetent cells were prepared as follows: E. coli MG1655ΔtyrR/pKD46 was grown overnight at 30° C. in LB medium containing ampicillin (100 mg/l), and the culture was diluted 100 times with 5 ml of SOB medium (Sambrook et al, “Molecular Cloning: A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press, 1989) containing ampicillin and L-arabinose (10 mM) [arabinose was used to induce the plasmid encoding the Red system genes]. The cells were grown with aeration at 30° C. to an OD₆₀₀ of ≈0.6 and then were made electrocompetent by concentrating 100-fold and washing three times with ice-cold deionized H₂O. Electroporation was performed using 70 μl of cells and ≈100 ng of the PCR product. The electroporation was done using a “BioRad” electroporator (USA, No. 165-2098, version 2-89) according to the manufacturer's instructions. The shocked cells were diluted with 1 ml of SOC medium (Sambrook et al, “Molecular Cloning. A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press, 1989), incubated at 37° C. for 2 h, and then spread on L-agar containing chloramphenicol (20 μg/ml) and grown at 37° C. to select Cm^(R) recombinants.

2. Verification of the pheA Gene Deletion by PCR

After 24 h of growth, colonies were tested for the presence of Cm^(R) marker by PCR using locus-specific primers P4 (SEQ ID NO: 12) and P5 (SEQ ID NO: 13). For this purpose, a freshly isolated colony was suspended in water (20 μl) and 1 μl of the obtained suspension was used for PCR. The temperature profile was as follows: the initial DNA denaturation at 95° C. for 5 min; then 30 cycles (denaturation at 95° C. for 30 s, annealing at 50° C. for 30 s, and elongation at 72° C. for 1 min 10 sec), and a final elongation at 72° C. for 5 min.

A few of the Cm^(R) colonies tested contained the required 2373-bp DNA fragment, confirming the substitution of the 1869-bp native region of the pheA gene by the hybrid tyrA-ssrA gene (see FIGS. 1 and 3). The resulting mutant strain was named MG1655 ΔtyrR ΔpheA::cat tyrA-ssrA.

3. Substitution of the Cat Gene in E. coli MG1655 ΔtyrR ΔpheA::cat tyrA-ssrA Strain with the Wild-Type pheA Gene.

To substitute the cat gene, a DNA fragment carrying the wild-type pheA gene was integrated into the chromosome of E. coli MG1655 ΔtyrR ΔpheA::cat tyrA-ssrA/pKD46 in place of the cat gene by the method described by Datsenko K. A. and Wanner B. L. (Proc. Natl. Acad. Sci. USA, 2000, 97, 6640-6645) which is also called “Red-mediated integration” and/or “Red-driven integration”, and is also described in Example 1 (FIG. 3).

The wild-type pheA gene was obtained by PCR using the chromosomal DNA of E. coli MG1655 as the template, and primers P4 (SEQ ID NO: 12) and P6 (SEQ ID NO: 14).

Primer P6 contains both a 30-nt region identical to the region located at positions from 90 to 119 of the ssrA gene having the stop codon TAA and a region complementary to the 19-nt region located at the 3′ end of the pheA gene.

Conditions for PCR were as follows: denaturation for 5 min at 95° C.; profile for 30 cycles: 30 sec at 95° C., 30 sec at 50° C., 1 min at 72° C.; final step: 5 min at 72° C.

The amplified DNA fragment was purified by agarose gel-electrophoresis, extracted using “GenElute Spin Columns” (“Sigma”, USA) and precipitated by ethanol. The obtained DNA fragment was used for electroporation and Red-mediated integration into the bacterial chromosome of the E. coli MG1655 ΔtyrR ΔpheA::cat tyrA-ssrA/pKD46 as described previously. The resulting E. coli strains MG1655 ΔtyrR tyrA-ssrA/pKD46 were selected on the minimal medium M9. The absence of L-phenylalanine in the medium enabled the selection of colonies that were Phe⁺ with prototrophic properties.

4. Verification of the pheA Gene Reconstruction by PCR

After 24 h of growth, colonies were tested for the presence of pheA gene by PCR using locus-specific primers P4 (SEQ ID NO: 12) and P5 (SEQ ID NO: 13). For this purpose, a freshly isolated colony was suspended in water (20 μl) and 11 of the obtained suspension was used for PCR. The temperature profile was as follows: the initial DNA denaturation at 95° C. for 5 min; then 30 cycles (denaturation at 95° C. for 30 s, annealing at 50° C. for 30 s, and elongation at 72° C. for 1 min 10 sec), and a final elongation at 72° C. for 5 min.

A few of the Phe⁺ colonies tested contained the required 1827-bp DNA fragment, confirming the substitution of the 2373-bp region of the cat gene by the pheA gene (see FIG. 2). The resulting mutant strain was named MG1655 ΔtyrR tyrA-ssrA/pKD46 (FIG. 4).

Then, to eliminate the pKD46 plasmid, two passages on L-agar with Cm at 42° C. were performed and the colonies which appeared were tested for sensitivity to ampicillin.

Thus, a strain with the mutant tyrA gene and missing the tyrR gene was obtained. This strain was named MG1655ΔtyrRtyrA-ssrA.

Example 2 Effect of ssrA Tagging of tyrA Gene on the L-Phenylalanine Production

Then E. coli strains MG1655 ΔtyrR tyrA-ssrA and MG1655 ΔtyrR were each cultivated at 37° C. for 18 hours in a nutrient broth, and 0.3 ml of each of these cultures was inoculated into 3 ml of fermentation medium having the following composition in a 20×200 mm test tube and cultivated at 32° C. for 72 hours with a rotary shaker.

After cultivation, the accumulated amounts of L-phenylalanine and L-tyrosine in the medium were determined by TLC. A thin-layer silica gel plate (10×15 cm) spotted with an aliquot (1-2 μl) of the culture broth was developed with a developing solvent (2-propanol:ethyl acetate:ammonia:water=16:16:3:9) and L-phenylalanine and L-tyro sine were detected with the ninhydrin reagent. The results of four tubes of fermentations are shown in Table 1.

The composition of the fermentation medium (g/l) was as follows:

Glucose 40.0 (NH₄)₂SO₄ 16.0 K₂HPO₄ 1.0 MgSO₄•7H₂O 1.0 MnSO₄•5H₂O 0.01 FeSO₄•7H₂O 0.01 Yeast extract 2.0 CaCO₃ 30.0

MgSO₄.7H₂O and CaCO₃ were each sterilized separately.

It can be seen from Table 1 that MG1655 ΔtyrR tyrA-ssrA caused the accumulation of a higher amount of L-phenylalanine and a lower amount of L-tyrosine as compared with MG1655 ΔtyrR.

Example 3 Effect of ssrA Tagging of pgi Gene on the L-Histidine Production

L-histidine-producing strains with the pgi gene tagged with peptides SEQ ID NO: 2 or SEQ ID NO: 15 were obtained. These two peptides differ by substitution of one of the terminal amino acid residues, either alanine or valine. These strains were obtained as follows. First, two variants of the pgi gene containing additional nucleotide sequences at the 3′-end of the gene coding for peptides SEQ ID NO: 2 or SEQ ID NO: 15 were obtained by PCR using chromosomal DNA of E. coli strain MG1655 and primers pgi1 (SEQ ID NO: 16) and pgi-LAA (SEQ ID NO: 17) or pgiLVA (SEQ ID NO:18), respectively. Primer pgi1 is complementary to the 5′ end of pgi gene and contains the SacI restriction site at the 5′-end thereof. Primers pgi-LVA and pgi-LAA contain regions of the 3′-end of the pgi gene minus the stop-codon, a region encoding for the short peptides SEQ ID NO: 2 or SEQ ID NO: 15, respectively, a stop-codon, and the XbaI restriction site at the 5′-end thereof. Second, two PCR fragments were separately treated with SacI and XbaI restrictases and ligated into the pMW119 plasmid which had been previously treated with the same restrictases. The resulting plasmids were named pMW-pgi-ssrA^(LAA) and pMwpgi-ssrA^(LVA), respectively.

Then, the wild-type pgi gene was deleted in the L-histidine-producing strain 80.

For this purpose, the DNA fragment containing the Cm^(R) marker encoded by the cat gene was obtained by PCR using primers P7 (SEQ ID NO: 19) and P8 (SEQ ID NO: 20), and plasmid pMW118-attL-Cm-attR as a template (WO 05/010175) (FIG. 2). Primer P7 contains both a region identical to the 36-nt region located at the 5′ end of the pgi gene and a region complementary to the 28-nt attR region of pMW118-attL-Cm-attR. Primer P8 contains both a region identical to the 36-nt region located at the 3′ end of the pgi gene and a region complementary to the 28-nt attL region of pMW118-attL-Cm-attR. PCR and electroporation of the obtained DNA fragment were performed as described above (see Example 1, section 1).

Deletion of the pgi gene was verified by PCR using locus-specific primers P9 (SEQ ID NO: 21) and P10 (SEQ ID NO: 22). For this purpose, a freshly isolated colony was suspended in water (20 μl) and 1 μl of this suspension was used for PCR. The temperature profile was as follows: the initial DNA denaturation at 95° C. for 5 min; then 30 cycles (denaturation at 95° C. for 30 s, annealing at 50° C. for 30 s, and elongation at 72° C. for 1 min 10 sec), and a final elongation at 72° C. for 5 min.

A few Cm^(R) colonies tested contained the required 1709-bp DNA fragment confirming the substitution of the 1651-bp native region of the pgi gene by cat gene. The resulting mutant strain was named 80 Δpgi.

The obtained strain 80 Δpgi was transformed with the plasmids pMW-pgi-ssrA^(LAA) and pMWpgi-ssrA^(LVA) as described above. The resulting strains were named 80 Δpgi/pMW-pgi-ssrA and 80 Δpgi/pMWpgi-ssrA^(LVA), respectively.

For mini-jar batch-fermentation, one loop of each strain 80, 80 Δpgi/pMW-pgi-ssrA^(LAA) and 80 Δpgi/pMWpgi-ssrA^(LVA) grown on L-agar was transferred to L-broth and cultivated at 30° C. with rotation (140 rpm) to reach an optical density of culture OD₅₄₀≈2.0. Then 25 ml of seed culture was added to 250 ml of medium for fermentation and cultivated at 29° C. for with rotation (1500 rpm). Duration of the batch-fermentation was approximately 35-40 hours. After the cultivation the amount of histidine which had accumulated in the medium was determined by paper chromatography. The paper was developed with a mobile phase:n-butanol:acetic acid:water=4:1:1 (v/v). A solution of ninhydrin (0.5%) in acetone was used as a visualizing reagent. The results are shown in Table 2. It can be seen from the Table 2 that strains 80 Δpgi/pMW-pgi-ssrA^(LAA) and 80 Δpgi/pMWpgi-ssrA^(LVA) caused the accumulation of a higher amount of L-histidine as compared with strain 80.

The composition of the fermentation medium (pH 6.0) (g/l):

Glucose 50.0 Mameno 0.2 of TN (NH₄)₂SO₄ 8.0 KH₂PO₄ 0.5 MgSO₄ 7H₂0 0.4 FeSO₄ 7H₂0 0.02 MnSO₄ 0.02 Thiamine 0.001 Betaine 2.0 L-proline 0.8 L-glutamate 3.0 L-aspartate 1.0 Adenosine 0.2

While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. All the cited references herein are incorporated as a part of this application by reference.

TABLE 1 Strain OD₅₄₀ Phe, g/l Tyr, g/l MG1655ΔtyrR 28.5 ± 0.6 0.13 ± 0.01 0.017 ± 0.002 MG1655ΔtyrR tyrA-ssrA 26.3 ± 0.4 0.46 ± 0.04 traces

TABLE 2 Amount of Yield per Strain OD₄₅₀ histidine, g/l glucose, % 80 (VKPM B-7270) 31.0 9.0 18.0 80 Δpgi 20.2 8.1 16.2 80 Δpgi/pMW-pgi-ssrA^(LAA) 23.2 10.7 21.4 80Δpgi/pMW-pgi-ssrA^(LVA) 22.8 10.6 21.2 

We claim:
 1. A recombinant L-amino acid producing Escherichia coli bacterium having a DNA comprising: A) a gene selected from the group consisting of tyrA, pheA, pgi, ilvE, ilvA, tdcB, sdaA, sdaB, argA, argG, proB, thrB, and combinations thereof, and B) a DNA fragment able to be transcribed and encoding the peptide of SEQ ID NO: 2, or a variant thereof consisting of a deletion, insertion, substitution, or addition of 1 amino acid as compared to SEQ ID NO: 2, wherein said DNA fragment of B) is attached to the 3′ end of said gene of A) and consequently enhances production of an L-amino acid.
 2. The bacterium of claim 1, wherein said bacterium is an L-phenylalanine producing bacterium.
 3. The bacterium of claim 1, wherein said gene is tyrA, which encodes the bifunctional enzyme chorismate mutase/prephenate dehydrogenase.
 4. The bacterium of claim 1, wherein said bacterium is an L-histidine producing bacterium.
 5. The bacterium of claim 1, wherein said gene is pgi which encodes a phosphoglucose isomerase enzyme.
 6. A method for producing an L-amino acid comprising cultivating the bacterium according to claim 1 in a culture medium, and isolating the L-amino acid from the culture medium.
 7. The method according to claim 6, wherein said L-amino acid is L-phenylalanine.
 8. The method according to claim 6, wherein said L-amino acid is L-histidine.
 9. The bacterium of claim 1, wherein the variant is the peptide of SEQ ID NO:
 15. 